MINERALOGICAL INVESTIGATIONS OF A METEORITE IMPACT BRECCIA FROM THE SERPENT MOUND ARCHAEOLOGICAL SITE, ADAMS COUNTY OHIO

Serpent Mound is an archaeological site located in Adams County, Ohio constructed by the Adena tribe between 1000 BCE and 700 CE. Located near the western edge of the Serpent Mound structure, is a unique astrobleme site based on bedrock geology, planar deformed quartz, and shatter cones. The geology at that location is complex, owing to the disturbance caused by the impactor. Detailed studies of the environmental mineralogy of the region are uncommon; such studies provide critical information for long-term archaeological site management and broader understanding of the geologic and environmental history of the region.

Rock samples were collected from an outcrop of impact breccia in the Silurian Tymochetee Formation northeast of the archaeological site. Those samples were analyzed using X-Ray Diffraction (XRD) and Scanning Electron Microscope (SEM). Results from the analysis indicate a wide range of minerals in the bedrock, composed of mostly dolomite (CaMg(CO₃)₂), some calcite (CaCO₃), and a minor amount of other mineral phases such as oxide minerals, sulfides, and minerals minor rare earth element (REE) bearing minerals. Euhedral kaolinite crystals approximately 5 micrometers in diameter were also commonly observed. The SEM investigation indicated the dolomite crystals to be anhedral to euhedral and possessed variable twinning textures and pitting. Natural asphalt or tar occurred as tacky infilling that completely filled the pores, which were commonly 10 micrometers to 50 micrometers in diameter. Energy Dispersive Spectroscopy (EDS) analysis indicated an appreciable amount of sulfur in the asphalt or tar material.

This is the first detailed mineralogical investigation of bedrock at the Serpent Mound archaeological site using SEM. The occurrence of natural asphalt or tar along with information on mineralogical constraints will provide a detailed reference for future environmental impact studies, particularly relating to road traffic, rock face preservation and geoarchaeological investigations of sediment and soil tied to site use.